Medullary nailing is frequently used for the treatment of lower leg fractures. The function of the medullary nail amounts to an internal splinting of the long bone. Depending on the method, a conventional medullary nail can consist of a tube or a number of metal rods, so-called cluster nailing. When a tubular nail is used, as is described for example, in EP-A1 0 332 857, the medullary area of the long bone must be prepared by milling. By means of this milling procedure, a close fit is achieved between the nail and the interior wall of the bone. This method provides optimum splinting of the bone. However, a pre-condition for the use of this method is that the bone fracture be able to accept axial and torsional forces.
The same restriction holds true in cluster nailing, in which the non-milled medullary area is filled with long metal rods. This type of splinting is heavily dependent on the skill of the individual surgeon, since it is not easy to fill the entire medullary space with metal rods.
If there is no bony support of the bone fracture itself (as because of fragmentation or bone defects), medullary bolting must be performed. For this type of medullary nailing, no fitting of the medullary area is necessary. Nevertheless, except in lengthy fragmented areas, the medullary area must usually be milled, since medullary nails that can be bolted have a certain minimum diameter and are somewhat stiffer.
Tubular medullary nails, for example, nails according to EP-A1 0 332 857, are shaped according to the milled medullary area. Tubular nails that cannot be bolted usually have a cloverleaf cross-section and a partial or complete longitudinal slot. The advantage of this is that the nail can be braced radially in the milled medullary area, with consequent increased stability of the fracture to impinging rotational forces. However, such bracing requires a dimensionally precise milling of the medullary area, to prevent the nail from becoming stuck when it is inserted. In bolting, radial prestressing is not done, since the fracture is secured by the transverse screws. The cross-section of such medullary nails is therefore approximately circular, so that it can be easily inserted into the milled medulla.
An attempt to hammer a tubular medullary nail into a non-milled medullary area requires a major expenditure of force, since the tibia cross-section does not correspond to the cross-section of the medullary nail. In this case a breaking of the tibia shaft often occurs, along with seizure of the nail.
To avoid the aforementioned problems, a thinner medullary nail can be selected. However, this is not always possible, since the mechanical strength of tubular pins is, by their very nature, limited. This limit can easily be reached, since this type of medullary pin possesses no rotation resistance; that is, in addition to the axially operative forces, rotational forces must also be borne fully by the transverse screws or pins.